The present invention refers to a method for the butt welding of products coming from the continuous casting of molten materials, and to the corresponding embodiment device, in particular usable in continuous-casting plants for the welding of bars, billets, blooms, slabs and the like.
With the continuous-casting technique, metallic elements are produced of a basically elongated shape having sections and length of the type referred to above. The length of these products coming from continuous casting varies and it is sometimes necessary to produce elements of greater length by joining a number of elements together by butt welding.
In the current art, welding methods are known which envisage the welding of the rear end of a cast element already set on the rolling line with the front end of another element coming from the casting device or else coming from a storage area. The aim of such welding methods is to gain time and accelerate production, which represents a very important factor for achieving high productivity of the casting line.
In addition to devices for butt welding, various devices for carrying out further processing or operations aimed at improving the quality of the finished product are generally integrated in the casting and rolling chains. Among such devices are cutting tools installed generally upstream of the welding device, for cutting the bars so as to make the surfaces of the ends plan and regular for better contact between the two elements to be welded. Then there are scaling tools, also set generally upstream of the welding device, for cleaning the surfaces of the bars prior to the rolling operation.
Downstream of the welding device, there are trimming tools for eliminating the surface irregularities produced by the welding operation. Other types of cutting tools or abrasive tools may be added according to the requirements.
A casting and rolling chain of this type is known from the Italian patent application UD96A000178, filed in the name of the present Applicant. The welding device, inserted in this casting chain, has two welding jaws which combine in a single element the functions of gripping, positioning and alignment of the cast products, as well as electric power supply functions for supplying the current needed for carrying out the operation of butt welding.
These known types of welding devices present various drawbacks which render their use problematical and make it necessary to carry out complex operations of preparation and finishing on the cast products, both before and after welding, in the phase prior to entry into the rolling mill.
In the welding device referred to, the gripping jaws, in addition to having a rather large surface of contact with the cast product, remain in contact with the surface of the bars, or of the other cast products mentioned above, for a relatively long time so as to complete the welding operation. From the contact between the jaws, which have a temperature lower than the temperature of the bars, and the surfaces of the latter, which are, instead, at a substantially higher temperature, there is produced, on the said bars, local cooling of surfaces of considerable dimensions, which may extend also rather deeply into the bar, generally referred to as xe2x80x9cblack patchesxe2x80x9d.
In known welding devices of the type described above, the gripping jaws in fact must normally have wide surfaces of contact with the bars, so as to be able to withstand the considerable forces involved in this phase of the casting and rolling process. The jaws must grip the ends of the bars to be joined and cause an energetic action of pressure to maintain sufficient contact during welding, as well as to compensate for the defects of shape of the bars, since the end surfaces of these frequently do not have a perfectly regular shape or else the bars are twisted along their length. During their gripping action, the jaws are consequently subject to heavy loads which they can sustain only if they are built of adequate size. Furthermore, the bigger the jaws, the more they can compensate for the surface irregularities of the cast product. On the other hand, the larger the dimensions of the jaws, the more heat transferred from the cast product, which can no longer be recovered from the considerable residual heat of the bar itself; this affects the size of the black patches.
On the other hand, the black patches, which remain on the bars after the welding operation, are one of the factors which give rise to adverse effects on the rolling chain in which the bars subsequently pass. In fact, in the case where these cast products are made of conventional carbon steel, there is a lower temperature in the area of the black patches and hence a mechanical resistance higher than that of the rest of the cast product, where the temperature is instead higher. During passage of the bars between the rolls of the rolling mill, every transition from a high-temperature, and mechanically more malleable, area, to a lower-temperature, and less malleable, area, such as the one of the black patches, causes violent impact in the rolls of the rolling mill which, in the long run, causes early wearing-out of the rolling mill. The presence of such areas of black patches moreover requires additional operations or processes to correct such residual defects.
In the other case, instead, in which the bars are made of special steel alloy, the possible presence of temperature differences between various areas of the bar itself may also induce thermo-mechanical transformations in its metallurgical structure.
On the other hand, to overcome the problem of the presence of black patches, it is not possible to increase the working temperature of the gripping jaws, which are in general made of high-conductivity metallic alloy, of the type Cu, Cuxe2x80x94Be, Cuxe2x80x94Bexe2x80x94Ni, Cuxe2x80x94Bexe2x80x94Fe, Cuxe2x80x94Wxe2x80x94Ni, Cuxe2x80x94Wxe2x80x94Fe, etc., and hence cannot work at excessively high temperatures. Consequently, it is necessary to provide forced cooling systems in the aforesaid jaws to prevent the risk of their mechanical collapse. In addition to this, their function as suppliers of electric current for welding contributes to increasing their temperature, which instead must be kept around a value of 500xc2x0 C. In this processing phase instead, the cast products have a relatively high temperature, which is approximately between 1000xc2x0 C. and 1200xc2x0 C. This fact, combined with a relatively long duration of contact between the jaws and the bars gives rise to black patches of excessive dimensions.
The aim of the present invention is thus to overcome the drawbacks referred to above presented by known welding devices.
A primary purpose of the present invention is that of overcoming the drawbacks referred to above by providing a method for butt welding and the corresponding welding devicexe2x80x94in particular for welding products of continuous castingxe2x80x94which present high productivity and reliability, contained costs, as well as guaranteeing an optimal welding quality, so as to reduce the processes on the product before its access to the rolling line.
The particular purpose of the present invention is to provide a welding device which enables a significant reduction in the time of contact of the electric power supplies with the surface of the cast product to be welded.
A further purpose of the present invention is to reduce substantially the dimensions of the areas of contact of the jaws with the surface of the cast product so as to reduce the dimensions of the so-called black patches, enabling complete reabsorption by means of the residual heat of the cast product.
Another purpose of the present invention is that of providing different mechanical elements for separating the functions for gripping the cast products from those of transmission of electric current for welding, so as to use the most appropriate materials for the respective functions in the fabrication of each element.
A further purpose of the present invention is to provide a welding device which may be introduced easily, i.e., without major adaptations, in the existing casting lines and which does not occupy more space than that occupied by devices of a type known.
These purposes are achieved by a welding device, in particular for butt welding of elements of a basically elongated shape, such as bars, billets, blooms or slabs produced by continuous casting, which comprises means for simultaneous conveying of at least two said elements along a processing line, means for gripping said at least two elements, means for the supply of electricity designed to be set in contact with the surfaces of said at least two elements, for carrying out a welding operation, characterized in that said gripping means and said electric power supply means are provided with respective positioning and actuation means, controlled by an actuation device designed to actuate the aforesaid gripping means and the aforesaid means for supplying electricity independently of each other to be set in contact with the surfaces of said at least two elements.
Thanks to this arrangement, the mechanical function is separate from the electrical one, with an important gain as regards the functionality of the welding device, so that each part is optimized in relation to its main functions.
According to a further aspect of the invention, a method is provided for butt welding of elements of a basically elongated shape, such as bars, billets, blooms or slabs produced by continuous casting which comprises the following steps:
a) producing by means of continuous casting at least two elements of substantially elongated shape,
b) conveying the first of said two elements to a rolling plant with a first feed speed and along a pre-set direction,
c) conveying the second of said two elements at a second feed speed higher than the one of the first of said two elements and along said pre-set direction,
d) setting the respective facing ends of the said two elements in mutual contact,
e) blocking said two elements by means of gripping jaws, set in a first operating position, so as to form a fixed block,
f) setting electric power supplies in contact with the side surfaces of said two elements,
g) supplying said electric power supplies with current for a pre-set time, to carry out mutual welding of the head of the said two elements and join them permanently together, making a single substantially elongated element,
h) detachment of said electric power supplies from the said single substantially elongated element,
i) detaching the jaws, in a second operating position, from said single substantially elongated element.
Thanks to the above method, formation of black patches is prevented and a product ready for rolling operation of optimal quality is obtained.